Here is the final installment of the PostScript article by Mark Barnes. …When lines join for example, there’s the question of how should they join at the point? If you “zoom in” on a line, the pixels are square, so a line is really something of a long, thin, rectangle, depending on your line thickness. When two of them meet at a 90 degree corner, for example, if the endpoint of their centerlines meet perfectly, then you get a little notch at the tip of the corner. Well you might not want that, you might want it to fill in that little notch. PostScript has an operator to define that. Or you might want a nicely rounded curve at the corner. It can do that too. PostScript is really powerful and you can describe pretty much damn near anything with it on a page.
Obviously it would be tedious to write a PostScript program every time you print a page, so that’s why we have printer drivers. Computer operating systems generally have their own way of describing graphic elements on a screen, so printing often has to interpret that and translate it to PostScript. That’s what PostScript printer drivers do.
Here is a quote from an article published TODAY that highlights some of the things I was telling you today about printing from Windows computers:
“Many people make PDFs in the application they are working in, and they print to Distiller or to something else, and what’s happening underneath is that the application is printing through the Windows print subsystem through a thing called the GDI [Graphic Device Interface] print subsystem. A print driver converts that to PostScript, and the PostScript gets converted to PDF.
However, there are a number of features that PDF supports that you can’t actually get into the PDF files through that route. An example of that is transparency. Applications like PowerPoint and Word in Office 2003 support transparency, but if you try to make a PDF from those, you don’t get that transparency information even though PDF can support that transparency.”
And finally, one last thing you will encounter when talking about PostScript: the RIP. A “RIP” is a Raster Image Processor. A printer must have a Raster Image Processor to take the PostScript code that defines what a human wants printed on a piece of paper, and map those lines and curves and graphics into the pixel space supported by a specific printer. An imagesetter may have a resolution of 3000 dots per inch. The RIP is a chunk of code and a computer in the printer that maps out what dots of the 3000 x 3000 dots in every square inch of your page get ink, and then sends that as a bunch of raw data to the print heads as they pass over the page.
So some RIPs are built into a printer. But since computers are pretty damn powerful these days, RIPping can also be done on a computer, and indeed, many companies write a software RIP that takes PostScript files and eats them, and do lots of calculations to figure out what dots to ink, and then spit that directly to a particular printer. They also build stand-alone devices that do this. Check out the Canon ColorPASS Z600 here:
http://www.canon.co.nz/products/copiers/controllers.html
It is used to turn an expensive Canon color laser copier into a powerful network printer, too. Look at this whole line of stand-alone RIPs here:
http://www.copydynamics.com/colorpass.htm
Neat, huh? -Mark
Well, there it is! I hope you got something out of it, I know I certainly did.
Take care everyone,
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